1. Technical Field
This invention relates in general to wireless communications and, more particularly, to a method and apparatus for reducing dynamic DC offsets in a wireless receiver.
2. Description of the Related Art
Dynamic DC-offset is a problem that affects most wireless receivers. This problem manifests itself as an additional time varying DC (direct current) offset to the received signal. There are many reasons for DC offset, and one of the most common is the mismatch between the receiver components. A Direct Conversion Receiver is a type of receiver that is more heavily affected than others by the DC offset problem.
FIG. 1a illustrates a basic block diagram of a Direct Conversion Receiver 10. In a Direct Conversion Receiver, the received RF (radio frequency) signal x(t)·cos(2πf0t+φ(t)), received at antenna 12, is moved to the baseband frequency by amplifying the received signal at LNA (low noise amplifier) 14 and mixing the amplified signal at mixer 16 with the output of a LO (local oscillator) 18 that has the same frequency, cos(2πf0t), as the carrier frequency of the incoming signal. The output of the mixer 16 is amplified by VGA (variable gain amplifier) 20. LNA 14 and VGA 20 are controlled by AGC (automatic gain control) 22, which determines the correct gain for each amplifier based on the power of the useful signal. Blocker signals (described in greater detail below) are detected by blocker detect circuitry 24 from the RF signal at the antenna; the baseband processor is notified regarding the existence of a blocker signal.
Due to coupling between the RF and LO inputs to the mixer 16, a part of the RF signal will go to the LO input to the mixer 16, and a part of the LO signal will go to the RF input to the mixer 16. Thus, the RF signal will not only be mixed with the LO signal, but also with itself (it will “self-mix”), and the same thing will happen to the LO signal. This will create an extra baseband component, commonly called DC offset because, when the RF signal is phase or frequency modulated, the result of self-mixing is a zero frequency term plus an extra term at twice the carrier frequency which can be easily filtered out. This DC offset will be added to the demodulated signal within the mixer, and thus cannot be filtered out.
The equation for the signal at the output of the mixer is:
            [                                    x            ⁡                          (              t              )                                ·                      cos            ⁡                          (                                                2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                +                                  ϕ                  ⁡                                      (                    t                    )                                                              )                                      +                                  ⁢                                  ⁢                              β            ·            cos                    ⁢                      (                          2              ⁢              π              ⁢                                                          ⁢                              f                0                            ⁢              t                        )                              ]        ·          [                        cos          ⁢                      (                          2              ⁢              π              ⁢                                                          ⁢                              f                0                            ⁢              t                        )                          +                  α          ·                      x            ⁡                          (              t              )                                ·                      cos            ⁡                          (                                                2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                +                                  ϕ                  ⁡                                      (                    t                    )                                                              )                                          ]        =          ⁢          ⁢                    1        2            ⁡              [                                            x              ⁡                              (                t                )                                      ·                          (                                                cos                  ⁡                                      (                                          ϕ                      ⁡                                              (                        t                        )                                                              )                                                  +                                  cos                  ⁡                                      (                                                                  4                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          0                                                ⁢                        t                                            +                                              (                        t                        )                                                              )                                                              )                                +                      β            ·                          (                              1                +                                  cos                  ⁡                                      (                                          4                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        0                                            ⁢                      t                                        )                                                              )                                      ]              +                  1        2            ⁡              [                                            α              ·                                                x                  ⁡                                      (                    t                    )                                                  2                                      ⁢                          (                              1                +                                  cos                  ⁡                                      (                                                                  4                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          0                                                ⁢                        t                                            +                                              2                        ·                                                  ϕ                          ⁡                                                      (                            t                            )                                                                                                                )                                                              )                                +                      α            ·            β            ·                          x              ⁡                              (                t                )                                      ·                          (                                                cos                  ⁡                                      (                                          ϕ                      ⁡                                              (                        t                        )                                                              )                                                  +                                  cos                  ⁡                                      (                                                                  4                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          0                                                ⁢                        t                                            +                                              ϕ                        ⁡                                                  (                          t                          )                                                                                      )                                                              )                                      ]            
After filtering, the output of the mixer can be described by:
      1    2    ⁡      [                            (                      1            +                          α              ·              β                                )                ·                  x          ⁡                      (            t            )                          ·                  cos          ⁡                      (                          ϕ              ⁡                              (                t                )                                      )                              +      β      +              α        ·                              x            ⁡                          (              t              )                                2                      ]  
As can be seen, there are three terms remaining after filtering. The first term is equal to the desired signal multiplied by a factor 1+αβ, which varies with time, because α and β depend on the coupling between the mixer inputs, the position of the mobile, and other factors. The second term is a pure DC offset term β due to the self-mixing of the LO, and the third term is an DC offset term due to the self-mixing of the RF input signal. If the RF signal has constant envelope, this term is another pure DC offset factor, but if it has a variable envelope, as is the case for an EDGE (Enhanced Datarates for GSM Evolution) signal, this term will have a certain bandwidth.
Assuming that α and β are small (as they should, because they represent a coupling phenomenon), the term αβ will be negligible and this source of interference can be ignored. Thus, the main interference will be due to the α and β terms. This term can change with time, because coupling depends on temperature and many other variables. But most of those variables change very slowly, and can be considered as “locally” constant; i.e., within a burst, or several burst, the variables will not change significantly. In reality, conditions which affect the α and β values are almost constant, but it will be assumed that they can change after several seconds.
The major problem is represented by the term α·x(t)2. This problem increases with higher levels of x(t).
The LNA 14 adds gain in order to reduce the overall noise figures of the receiver. This is very important when the power level of the useful signal is low in order to preserve a good Signal-to-Noise Ratio (SNR). This good SNR is necessary to insure that the receiver signal can be demodulated correctly by the receiver.
However, increasing the power level of the useful signal increases the SNR as well; thus, at higher levels of the useful signal, the importance of this initial gain from the LNA decreases accordingly. It can even become necessary to reduce the LNA gain to avoid saturating the reception chain or having a high DC-offset added to the useful signal. This is done by an AGC (automatic gain control) system, which traditionally has one input, the useful signal power (calculated by the DSP), and one output, the ideal gain for the following bursts. Thus, this AGC system is “uni-dimensional”.
Blocker signals (also referred to as “blockers” or “interferers”) complicate the dynamic DC offset problem. A blocking signal is a signal found at the mobile antenna 12 at a frequency different from the useful signal frequency. IN a GSM system, blocker signals usually belong to other users transmitting at different frequencies. Those signals are generally bursted, but could be continuous.
FIG. 1b illustrates the effects of a blocker signal on the useful signal. The blocker signal can create a static DC offset which is easily filtered out and a dynamic DC offset which is not easily filtered out. The dynamic DC offset may be caused either by a time-varying blocker envelope or if the blocker signal appears for only a portion of the receive burst of the useful signal. If there is a large blocker signal at the antenna, traditional AGC systems will exacerbate the dynamic DC offset problem, because using a high LNA gain also increases the effects of the blocker signal, thus increasing the dynamic DC-offset generated by the blocker signal.
One solution to this problem has been to use a DSP to track and compensate for the DC offset from the binary data. While this solution works in many instances, it has several drawbacks. First, it is computationally intensive, which has a non-negligible impact on the overall power consumption of a wireless device. Second, it will not work with all types of blocker signals, only constant or semi-constant envelope blocker signals.
Therefore, a need has arisen for a method and apparatus for compensating for DC offsets which works with all types of blocker signals without using excessive computational or power resources.